1. Field of the Invention
The present invention relates to an exhaust system for a motorcycle, in particular, for what is called an off-road type motorcycle.
2. Description of the Related Art
In a motorcycle used mainly for off-road driving, a vehicle-mounted component such as an engine is set at a high position in order to secure a large minimum ground clearance of a vehicle body. Since a seat height becomes high as a result, a proper place around a center of a seat is slightly recessed downward in order to improve a foot grounding property. Since the seat is mounted on a vehicle body frame forming a skeletal structure of the vehicle body, a vehicle body frame rear part (vehicle body frame portion positioned below a central part of the seat) is required to be low. In view of the above-described situations, the engine and the vehicle body frame are disposed close to each other.
Besides, an exhaust pipe connected to the engine is required to be installed in a vehicle body layout which is designed quite compactly. In a neighborhood of a vehicle body center in particular, in which a leg portion of a passenger is positioned, since it is necessary that the exhaust pipe does not touch the leg portion, the exhaust pipe is to be installed inside the vehicle body frame.
In a motorcycle for off-road driving described in Japanese Patent Application Laid-open No. 2001-278158 (“JP '158”)for example, an exhaust pipe (23) is disposed through the inside of a pivot plate (6). In a space formed inside the pivot plate (6), a vaporizer (24) and a rear cushion (19) and so on are disposed in addition to the exhaust pipe (23).
Further, in a motorcycle for off-road driving described in Japanese Patent Application Laid-open No. 2005-313671 (“JP '671”), a muffler structure of a pair of right and left mufflers is disclosed. One exhaust pipe (50) of these is extended to a side of a cushion unit (33) and branched into two around a rear of the cushion unit (33). One exhaust pipe (50) of the branched two runs across in a vehicle width direction through a space formed in the rear of the cushion unit (33) and in a front of a rear fender front part (42a), with silencers (31, 32) being disposed in right and left sides of a rear wheel (23).
However, in the exhaust system described in JP '158, the vaporizer (24) has a float chamber (24a) in a lower part thereof, necessarily resulting in a large size in a height direction and bulkiness. Besides, since a crankcase (29) is positioned directly below the float chamber (24a), it is practically difficult to secure a space to dispose another part or member under the vaporizer (24).
On the other hand, the exhaust system described in JP '671 has a branch structure in which a branch pipe runs between a cushion unit (33) and a rear fender (42). Therefore, a curvature radius of the branch pipe is small thereby leading to excessive exhaust resistance, so that an exhaust efficiency is reduced and it becomes quite disadvantageous for obtaining engine output. Further, the rear fender (42) is closely disposed in a rear space of the cushion unit (33). Thus, due to necessity of securing a space for passing the exhaust branch pipe, it becomes necessary to move a rear wheel (23) to backward or to move the cushion unit (33) to forward, so that a problem occurs such as a long and large wheel base and a reduced function of the cushion unit (33).